SOLAR-CYCLE LENGTH, GREENHOUSE FORCING AND GLOBAL CLIMATE

THE recent rise in global-mean surface air temperature is widely thought to be the result of increasing atmospheric concentrations of greenhouse gases1-3, but there are discrepancies between the predicted response of the atmosphere to this radiative forcing and the observed temperature changes1-5. Solar irradiance fluctuations have been proposed as a possible explanation for these discrepancies, and various solar properties (for example, radius6, smoothed sunspot number7 or cycle length8) have been suggested as proxies for solar irradiance variations in the absence of direct data. Here we model the effects of a combination of greenhouse and solar-cycle-length forcing and compare the results with observed temperatures. We find that this forcing combination can explain many features of the temperature record, although the results must be interpreted cautiously; even with optimized solar forcing, most of the recent warming trend is explained by greenhouse forcing.